Rain, Snow and Snowmelt

1
Rain, Snow and Snowmelt changes in recent
decades in western North America
Dan Cayan (1,2) Mike
Dettinger (2,1) Iris Stewart (1) Noah
Knowles (2)
1
SIO 2 USGS
Climate of wet days is crucial to Western
water. An advance to earlier spring runoff
timing from western snowmelt-dominated watersheds
is observed since 1950s. What caused it?
(precip timing, earlier melt, more rain, PDO
shift)?
2
Western US Annual Precipitation mountain
precipitation is crucial resource
Topography
3
In the West, many of the wettest days occur
during WARM storms when temperatures are near
rain/snow threshhold at middle (1000-2500m)
elevations
PPT (mm)
4
In the Sierra Nevada, 20 precip days provide 2/3
of annual precip
These days play out during a short season
when N Pac storms are active.
5
Sacramento Headwaters
Hydrological effect of warmer mid-elevation and
cooler high-elevation zones is seen in comparing
snowmelt runoff timing btw Sacramento (lower
elev) and San Joaquin (higher elev)
mm/day
San Joaquin Headwaters
Snowmelt
mm/day
Noah Knowles
6
If the Sierra warms by 2C Yosemite loses about
20 of snow covered area
Mike Dettinger
7
With climate warming, California would lose much
of its late spring Snowpack
By the end of the century California could lose
half of its late spring snow pack due to climate
warming. This simulation by Noah Knowles is
guided by temperature changes from PCMs
Business-as-usual coupled climate simulation.
Noah Knowles
8
Earlier spring flows last 2-3 decades
center time
Center Timing of snowmelt watersheds have
advanced by 1-5 weeks earlier across West
Iris Stewart
9
But in the West, Precip has come later!
Precipitation Timing Trends
Earlier streamflow in recent decades Has
occurred despite Precip coming later
Streamflow Timing Trends
10
Earlier streamflow
Atmospheric Circulation has driven warmer
spring temperature and ealier runoff
Later streamflow
Warming Trend Spring Iemp 1950-1997
11
Warm phase of PDO yields earlier snowmelt
12
But, streamflow timing correlations are even
stronger with spring temperature anomalies
TI is 4mo mean temp anom preceding during
month of CT
13
Partial correlations streamflow timing
fluctuations are quite strongly affected by
spring temperature (TI) Warm TI anomalies
Produce earlier CTs. PDO has evidently had
an influence, but not as much as TI
14
Examining temperature anomalies more carefully
1977-1998 minus 1950-1976 Tmin
dry days only
Dec-Feb
Winter and especially Spring temperatures were
warmer in the western United States during
1977-1998 relative to 1950-1976 But
this map reflects dry days (ppt0)
onlythis Suggests that there was Earlier
snowmelt
Mar-May
15
But, in addition to dry days wet days were
warmer during 1977-1998
Dec-Feb
Dry days
Wet days
16
Changes in synoptic patterns were involved in
the shift toward earlier spring flows a
different mix of wet days
Mike Dettinger
17
1977-1998 minus 1950-1976 Tmin wet days (gt6mm
precip)
Jan
Jan
Peak temp changes occurred in
March and April
Feb
Dec-Feb
Mar
Apr
May
18
Summary Rain vs. snow is crucial to water
issues in the West. In Californias Sierra
Nevada, only 20-30 days deliver most of the
years water. Timing of spring runoff advanced
1-3 wks in decades after 1977 r.e. decades
before Causes Winter and esp Spring temp
Certainly dry days but also wet days
were involvedthis suggests not only early
snowmelt but more immediate rainfall runoff
occurred. PDO contributed, but other climate
anomalies resulting in warmer winters spring
were evidently stronger influences. Since 1998,
PDO reversed but spring temp and spring flows
did not reverted Need more better
monitoring at mid-high elevations